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Acta Metall Sin  2018, Vol. 54 Issue (4): 537-546    DOI: 10.11900/0412.1961.2017.00353
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Crevice Corrosion of X70 and 3Cr Low Alloy Steels Under Supercritical CO2 Condition
Jianan ZOU, Xiaolu PANG, Kewei GAO()
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
Cite this article: 

Jianan ZOU, Xiaolu PANG, Kewei GAO. Crevice Corrosion of X70 and 3Cr Low Alloy Steels Under Supercritical CO2 Condition. Acta Metall Sin, 2018, 54(4): 537-546.

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Abstract  

With the exploitation of high pressure gas fields and the development of carbon capture and storage (CCS) techniques, the corrosion problem of steels under CO2 environment has been paid more and more attention. To transportation easier and cost reduction, CO2 in pipelines and containers is usually pressured to a high pressure, such as supercritical state. The supercritical CO2 corrosion environment includes the CO2-saturated aqueous phase and the water-saturated supercritical CO2 (SC CO2) phase. Moreover, corrosive ions such as Cl- usually exist in CO2 corrosion environment, which could accelerate the occurrence of corrosion. Low alloy steels, widely used as pipelines and construction materials in oil/gas and CCS industries, are susceptible to corrosion in the aggressive environment that contains high-concentration ions and acidic gases, especially to severe localized corrosion. In this work, the crevice corrosion behavior of 3Cr and X70 steels exposed in supercritical CO2-saturated 3.5%NaCl solution and NaCl solution-saturated supercritical CO2 phase was investigated. SEM, EDS and 3D laser microscopy were used to analyze the corrosion product scale on the steel surface. The results show that both the steels occurred crevice corrosion on the edge of crevice, but slightly occurred corrosion inside the crevice. The crevice corrosion occurred due to the galvanic effect of areas inside and outside the crevice. In supercritical CO2 phase, 3Cr steel exhibited a higher uniform corrosion resistance than X70 steel, while the crevice corrosion resistance of 3Cr steel was lower than that of X70 steel. The different crevice corrosion behaviors between X70 and 3Cr steels might be attributed to the synergistic effect of elements Cr and Cu on enhancing the crevice corrosion.

Key words:  crevice corrosion      supercritical CO2      low alloy steel      corrosion mechanism     
Received:  24 August 2017     
ZTFLH:  TG172.9  
Fund: Supported by National Key Research and Development Program of China (No.2017YFB0702100) and National Natural Science Foundation of China (No.51771026)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00353     OR     https://www.ams.org.cn/EN/Y2018/V54/I4/537

Fig.1  Schematic of the artificial crevice (PTFE—poly tetra fluoroethylene)
Steel C Si Mn Cr Mo Ni Cu Fe
X70 0.06 0.18 1.50 0.02 1.70 0.12 0.11 Bal.
3Cr 0.14 0.28 0.51 2.68 0.16 0.17 0.04 Bal.
Table 1  Chemical compositions of X70 and 3Cr steels (mass fraction / %)
Fig.2  Schematic of experimental setup
Fig.3  SEM images and corresponding magnification (inset) of corrosion product scales for different crevice areas of 3Cr steel immersed in aqueous phase containing supercritical CO2 for 240 h (a) inside (b) edge (c) outside
Fig.4  SEM images and corresponding magnification (inset) of corrosion product scales for different crevice areas of X70 steel immersed in aqueous phase containing supercritical CO2 for 240 h (a) inside (b) edge (c) outside
Fig.5  Cross-section morphologies and corresponding line scaning of the corrosion scales on the edge of crevice of 3Cr (a) and X70 (b) steels in aqueous phase containing supercritical CO2 for 240 h
Element 3Cr X70
Inside Edge Outside Inside Edge Outside
C 17.89 16.39 15.84 25.97 23.39 22.21
O 42.64 40.43 44.54 45.72 49.68 52.52
Fe 26.66 17.69 18.72 27.37 25.73 24.05
Cr 12.25 24.89 20.34 0.05 0.06 0.06
Cu 0.05 0.38 0.07 0.11 0.15 0.13
Table 2  EDS results of the corrosion product scale for different crevice areas of 3Cr和X70 steels in aqueous phase for 240 h (mass fraction / %)
Fig.6  Surface morphologies inside (a, c) and on the edge of crevice (b, d) of 3Cr (a, b) and X70 (c, d) steels after removing the scale in aqueous phase
Fig.7  SEM images and corresponding magnification (inset) of corrosion product scales for different crevice areas of 3Cr steel immersed in SC CO2 phase for 240 h

(a) inside (b) edge (c) outside

Fig.8  SEM images and corresponding magnification (inset) of corrosion product scales for different crevice areas of X70 steel immersed in SC CO2 phase for 240 h

(a) inside (b) edge (c) outside

Fig.9  Cross-section morphologies and corresponding line scaning of the corrosion scales on the edge of crevice of 3Cr (a) and X70 (b) steels in SC CO2 phase
Element 3Cr X70
Inside Edge Outside Inside Edge Outside
C 16.25 18.19 20.13 19.56 19.69 20.76
O 41.69 41.43 44.35 51.87 51.59 49.31
Fe 27.55 21.42 18.26 27.56 28.14 29.16
Cr 14.17 18.74 16.54 0.04 0.07 0.03
Cu 0.04 0.44 0.05 0.18 0.31 0.22
Table 3  EDS result of the corrosion product scale for different crevice areas of 3Cr和X70 steels in SC CO2 phase for 240 h (mass fraction / %)
Fig.10  Surface morphologies inside (a, c) and on the edge of crevice (b, d) of 3Cr (a, b) and X70 (c, d) steels after removing the scale in SC CO2 phase
Fig.11  3D surface morphologies around the edge of crevice of 3Cr (a, b) and X70 (c, d) steels in aqueous phase (a, c) and in SC CO2 phase (b, d)
Fig.12  Average corrosion depths around the edge of crevice of 3Cr (a, b) and X70 (c, d) steel in aqueous phase (a, c) and in SC CO2 phase (b, d)
Fig.13  General and local corrosion rates of 3Cr (a, b) and X70 (c, d) steels in aqueous phase (a, c) and in SC CO2 phase (b, d)
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